This invention relates to articles coated with an environmental resistant coating, and, more particularly, to coated articles made of high temperature alloys for operation at high temperatures in a strenuous operating environment.
Gas turbine engine components operating in the hotter sections of such an engine, for example the turbine section, experience wide temperature extremes in an oxidizing and corrosive environment. Therefore, it has been a practice to apply to surfaces of such components a variety of types and combinations of high temperature protective coatings widely reported in the art. The coatings not only protect the component surface from the environment but also afford an engine designer the opportunity to improve efficiency of the engine by increasing operating temperatures.
Typical gas turbine engine component coatings include oxidation and corrosion resistant coatings including aluminum, forms of which are called diffusion aluminide coatings, on an article substrate. Such coatings are applied alone or in combination with an outer thermal barrier coating (TBC), for example of the ceramic type. However, as a result of cyclical operation under strenuous conditions, such protective coatings are subject to degradation. It is believed that such degradation of a protective coating or coating combination can result from inter-diffusion of coating and substrate elements along relatively direct grain boundaries toward the coating surface. In addition to the chemical degradation, cyclical exposure of the substrate-coating system resulting in growth of a thermally grown oxide (TGO) layer within the coating system can result in a condition sometimes referred to as xe2x80x9crumplingxe2x80x9d. Such a condition has been observed to be more severe at a junction of xe2x80x9cstraight-throughxe2x80x9d grain boundaries and the TGO. One typical reported coating combination or system comprises a substrate, such as a Ni base superalloy, having a single phase aluminide type coating diffused into the substrate, and a ceramic type TBC over the diffused aluminide layer. The aluminide coating acts as a bond coat for the TBC and an oxidation resistant coating for the substrate. The single phase aluminide coating generally is grown with grain boundaries extending directly, in a relatively straight path, between a diffusion zone of the coating at the substrate and the outer TBC. With this as-deposited microstructure, having relatively simple direct grain boundaries, it is believed that at least one element from the substrate and/or the diffusion zone can diffuse easily and directly to an interface between the TBC and the underlying, additive bond-type coating. If the element that diffuses to the bond coat/TBC interface forms a non-protective oxide, this occurrence can result in spalling of the TBC. Also, such a microstructure is susceptible to oxygen penetration from the environment along the grain boundaries, causing oxidation and stressing the coating. This also can result in spallation. In addition, such microstructure can be relatively weak and prone to surface displacement during thermal cycling to elevated temperatures. These surface displacements can cause damage to the TGO and eventual spallation of the TBC.
The present invention, in one form, provides a coated article comprising a substrate, and a coating combination on the substrate. The coating combination includes a coating diffusion portion at the substrate, an outer coating portion outwardly from the diffusion portion and a coating stabilizing portion between the coating diffusion portion and the outer coating portion. The coating stabilizing portion includes a microstructure having a plurality of grains with grain boundaries between adjacent grains defining a plurality of labyrinthine paths through the stabilizing portion between the coating diffusion portion and the outer coating portion.
In another form, the present invention provides a method for making a coated article in which such a coating stabilizing portion is provided between the diffusion portion and an outer coating portion.